Chemical equilibrium alternate forms

An isotope effect (either kinetic or equihbrium) resulting from reactions in which the different isotopes occupy chemically equivalent alternative reactive sites within the same molecular entity. In such cases, isotopicaUy distinct products are formed. See Intermolecular Isotope Effect Kinetic Isotope Effect Equilibrium Isotope Effect lUPAC (1979) Pure and Appl. Chem. 51, 1725. 

There is an alternative explanation for the conductivity enhancement of semiconductors when doped. We consider an analogue taken fi-om chemical equilibrium. The intrinsic conductivity of semiconductors can be considered to be analogous to the intrinsic conductivity of pure water given by its self dissociation. In dissociation, the ions HsO and OH are formed, however in a very low concentration. By apphcation of the law of mass action it follows that the product of their concentrations is a constant in equilibrium ... 

If one of the constituent mixt ures, a or h, in a reciprocal system is ideal (Wq] = Wq2 = O) or if its solution properties are well-known, equation (39 can he of considerable use in determining the solution properties of the other (coexisting) phases. Specifically, by equilibrating a and b experimentally at fixed temperature and pressure for a variety of bulk compositions, it is possible to establish numerous chemical tie lines between two coexisting members of each solution series, a and b. (At equilibrium, the compositions and proportions of the coexisting phases must balance through the bulk composition in accordance with the lever rule). If we know and W 2 ov can assume a particular kind of mixing behaviour for solution a, it is convenient to write equation (39) in an alternate form ... 

While enzymes, like organic catalysts, speed the approach to chemical equilibrium by providing an alternative pathway having a lower energy of activation, they can be distinguished by their enormous capabilities for stereospecific recognition and for rate acceleration. These faculties provide for the brisk conduct of the myriad chemical reactions, termed intermediary metabolism, that form the fabric of life s processes. 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

However, these potentials do not yet express the second law in the form most convenient for chemical applications. Open laboratory vessels exposed to the temperature and pressure of the surroundings are subject neither to constraints of isolation (as required for entropy maximization) nor to adiabatic constant-volume conditions (as required for energy minimization). Hence, we seek alternative thermodynamic potentials that express the criteria for equilibrium under more general conditions. 

When liquid and gas phases are both present in an equilibrium mixture of reacting species, Eq. (11.30), a criterion of vapor/liquid equilibrium, must be satisfied along with the equation of chemical-reaction equilibrium. There is considerable choice in the method of treatment of such cases. For example, consider a reaction of gas A and water B to form an aqueous solution C. The reaction may be assumed to occur entirely in the gas phase with simultaneous transfer of material between phases to maintain phase equilibrium. In this case, the equilibrium constant is evaluated from AG° data based on standard states for the species as gases, i.e., the ideal-gas states at 1 bar and the reaction temperature. On the other hand, the reaction may be assumed to occur in the liquid phase, in which case AG° is based on standard states for the species as liquids. Alternatively, the reaction may be written... 

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